An essential and recurring theme during vertebrate organogenesis is the early specification of single or small groups of cells (i.e., progenitors) that, later in development, give rise to specific organ systems. For example, a subset of yolk sac cells is the first source of embryonic hematopoietic cells and specific neural crest lineages give rise to significant portions of the peripheral nervous system. Little data exist, though, on the earliest cells that give rise to the vertebrate kidney. Identification and characterization of kidney progenitor cells is important because the vertebrate kidney is regenerative, but the molecular mechanisms of nephric regeneration are largely unknown. It is possible that regenerative cells of the adult kidney are developmental^ related to the earliest kidney progenitor cells in the embryo. This proposal aims to identify the earliest embryonic cells that give rise to the vertebrate kidney (Aim 1) and determine the role of Lim1 and Pax8 during kidney development (Aim 2). We will test the hypothesis that Lim1 and Pax8 are regulators of intermediate mesoderm progression to nephric restricted tissue. Moreover, we will determine the role the chemical compound, 4-(phenylthio)butyric acid plays in influencing nephric tissue specification not only in zebrafish embryos but also in organ culture (Aim 3). We use both zebrafish and Xenopus embryos because their genetic and embryological features complement each other and those of mammalian models to permit experimental investigation of early kidney development in a vertebrate system. The Aims outlined in this proposal combine experimental embryology, molecular biology, and state-of-the-art microscopy to identify kidney progenitor cells. Results of these investigations are directly translatable to efforts in other vertebrates, particularly humans for delineating molecular events that can influence kidney-restricted progenitor cell differentiation. The relevance of this research to public health is the vertebrate kidney is a complex homeostatic organ that functions to detoxify blood, maintain ion and water equilibrium, and regulate hormone release. The physiological consequences of abnormal kidney formation or function are frequently fatal, with dialysis and organ transplantation the only long-term treatments for kidney disease. Future strategies to fight kidney disease must rely on a fundamental understanding of the earliest events that lead to the formation of the kidney. [unreadable] [unreadable] [unreadable]